SKENEA. 153 



animals, which not only carry their houses on their 



backs, ''but have to see through them as they glide 



on their marshy way." 

 VI. Skenea includes another set of minute littoral mol- 

 luscs, resembling Rissoa, excepting that their shells are flat- 

 tened. The species found on the shores of our islands 

 are — 

 S. planorbis : smooth, with wide umbilicus, very much like 



a microscopic Ammonite. 

 S. nitidissima : polished, with spire concave. 

 S. rota : wheel-like, having ribs radiating from the centre of 



the spiral disc, above and below. 

 8. clivisa : like 8. planorhiSj but striated on the under side. 

 8. Cutleriana : not quite so much depressed ; striated above 



and below. 

 8. lavis : shaped like 8. Cutleriana, but smooth, excepting 



in the umbilicus, where it is spirally striated. 

 8henea planorbis congregates in great numbers under 

 stones, at low water. 



